In 1989, a main causative virus of non-A non-B posttransfusion hepatitis was found and named hepatitis C virus (HCV). Since then, several types of hepatitis viruses have been found besides type A, type B and type C, wherein hepatitis caused by HCV is called hepatitis C.
The patients infected with HCV are considered to involve several percent of the world population, and the infection with HCV characteristically becomes chronic.
HCV is an envelope RNA virus, wherein the genome is a single strand plus-strand RNA, and belongs to the genus Hepacivirus of Flavivirus (from The International Committee on Taxonomy of Viruses, International Union of Microbiological Societies). Of the same hepatitis viruses, for example, hepatitis B virus (HBV), which is a DNA virus, is eliminated by the immune system and the infection with this virus ends in an acute infection except for neonates and infants having yet immature immunological competence. In contrast, HCV somehow avoids the immune system of the host due to an unknown mechanism. Once infected with this virus, even an adult having a mature immune system frequently develops persistent infection.
When chronic hepatitis is associated with the persistent infection with HCV, it advances to cirrhosis or hepatic cancer in a high rate. Enucleation of tumor by operation does not help much, because the patient often develops recurrent hepatic cancer due to the sequela inflammation in non-cancerous parts. In addition, there is a report on the involvement of HCV infection in dermatosis such as chronic urticaria, lichen planus, cryoglobulinemic purpura and the like (The Japanese Journal of Dermatology, Vol. 111, No. 7, pages 1075-1081, 2001).
Thus, an effective therapeutic method of hepatitis C is desired. Apart from the symptomatic therapy to suppress inflammation with an anti-inflammatory agent, the development of a therapeutic agent that reduces HCV to a low level free from inflammation and that eradicates HCV has been strongly demanded.
At present, a treatment with interferon is the only effective method known for the eradication of HCV. However, interferon can eradicate the virus only in about one-third of the patient population. For the rest of the patients, it has no effect or provides only a temporary effect. In recent years, polyethylene glycolated interferon has been put to practical use, and enhanced effects and reduced side effects have been achieved. However, complete response rate still remains at a low level, and therefore, an anti-HCV drug to be used in the place of or concurrently with interferon is awaited in great expectation.
In recent years, Ribavirin (1-β-D-ribofuranosyl-1H-1,2,4-triazole-3-carboxamide) has become commercially available as a therapeutic agent for hepatitis C, which is to be used concurrently with interferon. It enhances the efficacy of interferon but only to a low efficacy rate, and a different novel therapeutic agent for hepatitis C is desired.
Also, an attempt has been made to potentiate the immunocompetence of the patient with an interferon agonist, an interleukin-12 agonist and the like, thereby to eradicate the virus, but an effective pharmaceutical agent has not been found yet.
In addition, the inhibition of HCV growth, wherein HCV-specific protein is targeted, has been drawing attention these days.
The gene of HCV encodes a protein such as serine protease, RNA helicase, RNA-dependent RNA polymerase and the like. These proteins function as a specific protein essential for the growth of HCV.
One of the specific proteins, RNA-dependent RNA polymerase (hereinafter to be also briefly referred to as an HCV polymerase), is an enzyme essential for the growth of the virus. The gene replication of HCV having a plus-strand RNA gene is considered to involve synthesis of a complementary minus-strand RNA by the use of the plus-strand RNA as a template and using the obtained minus-strand RNA as a template, amplifying the plus-strand RNA. The portion called NS5B of a protein precursor, that HCV codes for, has been found to show an RNA-dependent RNA polymerase activity (EMBO J., Vol. 15, pages 12-22, 1996), and is considered to play a central role in the HCV gene replication.
Therefore, an HCV polymerase inhibitor can be a target in the development of an anti-HCV drug, and the development thereof is eagerly awaited. However, an effective HCV polymerase inhibitor has not been developed yet, like in other attempts to develop an anti-HCV drug based on other action mechanisms. As the situation stands, no pharmaceutical agent can treat hepatitis C satisfactorily.
The following describes known compounds comparatively similar to the present invention.
As anti-HCV agents, which are thieno[3,2-b]pyrrole derivatives, the following compound e and the like are known (WO2005/23819, page 71, Table 1, compound No. 113).
While this reference describes that the compound and the like have HCV polymerase inhibitory activity, it does not disclose the compound of the present invention, nor does it contain a description suggesting that the compound of the present invention has an anti-HCV activity.

As the thieno[3,2-b]pyrrole derivative, moreover, the following compound a etc. are also known (Chemistry of Heterocyclic Compounds, pages 1133-1136, 1976, compound IIc (page 1134, Table 1)).
This reference includes descriptions relating to the synthetic methods of thieno[3,2-b]pyrrole derivative, but does not disclose the compound of the present invention, nor does it contain a description relating to use as a pharmaceutical product. In addition, it does not contain a description suggesting such use.

As tetracyclic fused heterocyclic compounds having known pharmaceutical use, the following compound b and the like are known, and synthetic methods of said compounds usable as central nervous system agents are known (Bollettino Chimico Farmaceutico, Vol. 120, No. 2, pages 102-107, 1981).

However, this reference does not disclose the compound of the present invention, not to mention use of the compound of this reference as an antiviral agent or a description suggestive thereof.
As tetracyclic fused heterocyclic compounds for use other than pharmaceutical use, the following compound c and the like are known, wherein its synthetic method is described (J. Org. Chem., Vol. 66, No. 2, pages 412-420, 2001, Table 3, No. 19 (page 415)).

A different reference discloses the following compound d etc., wherein its synthetic method is described (Organic Letters, Vol. 4, No. 8, pages 1355-1358, 2002, Table 1, No. 17 (page 1357), Scheme 4 (page 1356)).
